1. Field of the Invention
This invention relates generally to compact disc injection molding apparatus and more particularly to hot sprue bushings for use in molding compact discs.
2. Description of the Prior Art
Compact discs, which were first developed by Philips and Sony in the early 1980's, are now a well-established form of medium for recording a variety of retrievable information. Their popularity and success stem from their compactness, convenience, competitive cost, and high information storing capacity.
Physically, such discs are thin, flat, circular objects approximately 120 mm in diameter by about 1.2 mm thick. They are provided with a central hole about 15 mm in diameter for purposes of mounting them for rotation in various devices capable of reading and writing information to and from them. Typically, audio, video or other data is recorded in digital form on one surface of the disc with the opposite remaining free for carrying information about the contents of the recorded information.
For enabling writing and recording information, a compact disc (CD) comprises a clear plastic layer over a thin reflective aluminum surface several micrometers thick. Data, which may be any of a variety of types, is stored on the disc in binary code; the `zeros` of the code being dents or pits in the plastic surface while the `ones` are represented by smooth plastic. When playing the disc, a laser beam scans the disc surface as it rotates and is reflected back only by the `one` areas. Reflected light pulses are picked up by a photodetector which converts them into a digital electrical signal that can then be further processed for subsequent use in various playback devices appropriate for the type of information recorded in the disc.
While very popular for audio recording, CDs also can be used for storing video information, which applies the same technology. These have advantages over magnetic tape by virtue of being easier to replicate and randomly access.
CDs are also used to store large amounts of computer information. Here, CD-ROM (compact disc with read-only memory) is the most common format, but other forms of disc are available that allow data to be written on the disc as well as read.
Recent applications for CD use include the photo-CD, which records still images for viewing on a television screen or reproduction as hard copy, and the multimedia CD, which holds pictures, sound, and text on one disc and can play them back through television receiver and hi-fi equipment or appropriately configured computer systems.
While CDs are physically robust, they require great care in their fabrication to provide their necessary physical attributes and the proper encoding of recorded information. Moreover, all of this must be done in a highly competitive environment where large product volumes are routine.
Current fabrication depend on the use of high volume injection molding systems and related coating equipment for forming the reflective aluminum surface. However, the molding equipment in known use employs cold sprue systems or bushings which lead to undesirable material losses and high defect levels, especially birefringence and black and white spots, all of which would lead to intolerable reading errors.
In view of the known art, there continues to be a demonstrable need for more effective ways of providing significantly improved compact discs having fewer manufacturing defects and thus higher yields. And, it is a primary object of this invention to fulfill this need.
Another object of the present invention is to provide injection molding arrangements by which compact discs can be molded with hot sprue bushings to reduce material losses and defect levels, the viewable surfaces of flat containers may be exploited to provide views of different information by observing those surfaces from slightly different perspectives.
Yet another object of the present invention is to provide optical arrangements by which encoded information may be provided on a viewable surface.
Yet another object of the present invention is to provide optical viewing arrangements for compact disc containers that can be implemented by relatively minor modification of currently existing molds for fabricating them.
Still another object of the present invention is to provide optical arrangements by which pseudo-dynamic information may be provided to an observer of a flat container box.
Other objects of the invention will be apparent and will appear hereinafter in the following detailed description when read in connection with the drawings.